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Research aims for accurate peanut maturity measures

The indeterminate nature of a peanut plant – and the fact that it puts on its fruit underground – add to the complexities of determining when to dig a crop. But a lot can be won and a lot can be lost based on your decision, says Wilson Faircloth, research agronomist with the National Peanut Research Laboratory in Dawson, Ga.

“You can get a lot of gain in yield sometimes in the final 20 days of a peanut’s production cycle,” says Faircloth. “Some peanuts put on 30 percent of their growth in the last two to three weeks of the season – that’s huge. When it comes to economics, we’ve got to make the best decision possible when it comes to digging peanuts.”

According to the hull-scrape method, approximately 140 days can be an optimal peanut digging time. “But on some of these varieties, there might be a little bit left on the table yet that we didn’t account for. That’s why we’re looking at new methods and ideas to make sure we’re at the top of the curve and not at the bottom end of it,” says Faircloth.

A few late-maturity cultivars never quit gaining in value when researchers stopped sampling at 154 days, he says. When those peanuts were dug at about the 145-day mark, money was left in the field, says Faircloth, because the maturity prediction date was not accurate.

When to dig

“When do I dig?” remains a big question for growers, he says. “We have the hull-scrape method to help determine when we dig. And a lot of times, we start digging whenever our neighbors start. But the hull-scrape method is a very accurate tool, and sometimes we determine when to dig by looking at the calendar. You look at the day you planted and count the days until maturity.”

The truth is, neither of these methods by itself is a good idea. “But we can put them together to formulate an educated guess on the right time to dig peanuts.”

Since peanuts are underground, growers can’t just ride by a field, take a quick look, and tell if they’re ready to dig. “And you can’t put them back once you dig them, so you have to be spot on.”

With the hull-scrape method, growers can place the pods on a profile board according to color class. Whenever there are accumulations in the black regions, you can predict a pretty accurate digging date, says Faircloth. A uniform curve on the board indicates that the pods all were set at one time over the course of a season.

Two crops

Sometimes, the board indicates that you have two crops of peanuts. “You can have a crop that put on early – as it should have – and a crop that put on later. More than likely, you had a weather event that caused a decrease in pod set, and then conditions improved later in the season. This can cause a farmer to get into a guessing game, wondering how much of the crop he can afford to wait on, and how much might he lose if waits for the late crop to mature,” says Faircloth.

The 2010 crop year has seen some good examples of this due to extreme heat.

“Peanuts are indeterminate. In other words, they don’t have a time at which they must fruit and can’t fruit. They can fruit and flower throughout the season as long as conditions are right. Contrast that with corn. When corn comes out of the ground and is 10 to 20 inches tall, its yield is predetermined. You can take away yield and add to the development of that yield, but its yield potential is pretty well capped at a very early stage.

“With peanuts and cotton, they can quit fruiting when the weather gets hot. Then, when the weather improves, they can go back to fruiting. They fruit continuously, and that’s what makes it so difficult and why we get split crops and uneven distributions.”

In one trial this year, two varieties – Georgia Green and AP-4 – gained 20 percent in yield over a period of eight days, says Faircloth. As they increased in maturity, they greatly improved in yield.

Profile board

The profile board is a very good predictor of maturity, he says, but it has its faults. Sampling errors, he adds, can be a problem. “Often, you overestimate maturity because you’re getting the biggest, most mature peanuts, and you’re not accounting for all of the peanuts in the field.”

Time also can be an issue. “I do about 500 of these boards every fall as part of my research, and it takes 15 to 20 minutes to do this right. You have to get the pod, look at it, and sometimes feel. At times, you have to go by the touch and consistency – whether it’s soft and watery or hard and firm. Also, the pod blasting must be available to you.”

Research is not focused on doing away with the profile board, he says, but rather to find some methods that make it easier to do or that might eventually take its place and be accurate and reliable.

“Some peanuts just don’t fit the board. AP-3 never turned black. The profile board was developed over 30 years ago, and it’s still very relevant, but we need to come up with some updates.”

Researchers are working on a growing degree day method for determining maturity, says Faircloth. This is a method commonly used in cotton and known as DD60s. It’s the amount of heat units received by a plant.

Heat Units

“There’s quite a good correlation with DD60s on peanuts – you just have to make a few modifications. It’s very accurate and a very good correction factor for the profile board. It’s not a stand-alone method to predict maturity, but maybe with some other features built in, it very well could be.”

With peanuts, the cotton DD60 becomes a DD56, he says. The DD56 is a threshold, assuming that below 56 degrees F, a peanut plant is not engaged in high growth such as putting on flowers and pods

A DD56 would be calculated by adding the maximum and minimum temperature, divided by two, minus 56. You add up DD56s to determine the heat unit for a given period of time.

“The important part of this is that you have to add in the water. It doesn’t take just heat to mature a peanut – it also takes water, and you must have both together. It’s a pretty good tool for determining peanut maturity.”

This method was evaluated last year, and every variety tested peaked at 2,500 growing degree days, plus or minus eight to 10 growing degree days, depending on the variety, says Faircloth.

In July or August, when daytime temperatures are in excess of 95 degrees F and low temperatures at night are in the 70s, a peanut plant can accumulate about 15 growing degree days per day.

“We’ve got a lot more research to do with this, but it’s a great correction factor when you’re using the hull-scrape method. You can look at the distribution on your board, and then look at the heat accumulation and make a good determination.”